Lithium secondary batteries have the advantages of high energy density, little self-discharge, excellent long-term reliability and the like, and are therefore broadly put to practical use as batteries for small-size electronic devices such as notebook computers and cellular phones. In recent years, high functionalization of electronic devices and utilization of lithium secondary batteries for electric vehicles have progressed, and the development of higher-performance lithium secondary batteries is thus demanded.
At present, carbon materials are common as negative electrode active materials for lithium secondary batteries, and various types of carbon materials are proposed in order to improve the battery performance.
As the carbon materials, there are known high-crystallinity carbon such as natural graphite and artificial graphite, low-crystallinity carbon such as graphitizing carbon (soft carbon) and non-graphitizing carbon (hard carbon), and amorphous carbon. Graphite, which is a high-crystallinity carbon, is known to be excellent in the reactivity with Li ions and is capable of providing a capacity near to its theoretical capacity value. By contrast, the high-crystallinity carbon causes a decrease in the cycle characteristics due to the degradation of the electrolyte solutions, since it is liable to react with propylene carbonate (PC) often used as a solvent of electrolyte solutions. Low-crystallinity carbon and amorphous carbon, which have just a higher theoretical capacity value than that of graphite, are low in the reactivity with Li ions, then need charging for a long time, and have a lower capacity value per unit time than graphite. By contrast, the reactivity with PC is low and the degradation of electrolyte solutions is little. Then, there has been proposed a composite carbon material which includes the combination of a graphite and an amorphous carbon (including low-crystallinity carbon).
For example, Patent Literature 1 discloses a negative electrode active substance in which an amorphous carbon is adhered on the surface of graphite particles. The Patent Literature discloses that in order to improve the close adhesivity of the graphite particles with the amorphous carbon, the graphite particles are oxidatively treated to form oxygen-containing functional groups on the graphite particle surface, and that the surface thereof is roughened. For example, Patent Literature 1 discloses a method including oxidizing graphite particles with air at a temperature of 200 to 700° C., adhering an alkali on the graphite particle surface, and thereafter, subjecting the graphite particles to a heat treatment at 300° C. to 700° C.